1. Field of the Invention:
This invention relates to metallurgy and to zirconium and hafnium metallic compositions, and in particular to a very high purity reactive metal sponge material.
2. Description of the Prior Art:
In the commercial production of zirconium and hafnium metal, the ore is generally initially subjected to a chlorination step which produces a relatively impure, hafnium-containing zirconium tetrachloride and by-product silicon tetrachloride (which by-product is relatively easily separated). The hafnium and zirconium containing material is then subjected to a number of purifying operations and also a complex hafnium separation operation. These operations result in purified oxides of zirconium and hafnium, which, of course, are maintained separate. The purified oxides are separately chlorinated. Zirconium and hafnium are commonly reduced from the chloride by means of a reducing metal such as magnesium. At the present time, the commercial processes are batch-type processes. U.S. Pat. No. 3,966,460, for example, describes a process of introducing zirconium tetrachloride vapor onto molten magnesium, with the zirconium being reduced and traveling down through the magnesium layer to the botton of the reactor and with the by-product magnesium chloride being periodically removed. In the commercial processes, however, the by-product salt (e.g. magnesium chloride) remains in the reduction vessel until the batch is completed and cooled. The salt and metallic sponge (zirconium or hafnium) are then removed from the reduction vessel. The metallic sponge (containing remaining salt and some remaining excess reducing metal) is then placed in a distillation vessel for removal of the remaining salt and magnesium by high temperature vacuum distillation. Generally this intermediate product sponge contains 1,000-5,000 ppm (and more typically 2,000-3,000 ppm by weight) of total impurities, including generally about 500-1,000 (and more typically 700-1,000) ppm oxygen, 300-800 ppm iron, 30-70 ppm aluminum, 1-5 ppm uranium, and 10-20 ppm phosphorous.
The sponge material is generally crushed, screened, and pressed into electrodes for melting (alloying elements are often added to the vacuum arc melting electrode). The crushed sponge material is particulate and of minus 1/4 inch, and plus 16 mesh in size. Typically the material is double vacuum arc melted to provide ingots which are then further fabricated into various shapes. Most of the zirconium currently is used to produce Zircaloy.
Commercial nuclear reactors generally have used Zircaloy tubes as cladding material to contain the uranium dioxide fuel. Generally a Zircaloy ingot is processed into a so-called "trex" and pilgering operations are used to reduce the trex inside diameter and wall thickness to size.
Ultrapure zirconium has been proposed for a liner for the inside surface of Zircaloy tubing which is used as cladding for nuclear fuel and is described in, for example, U.S. Pat. No. 4,372,817 to Armijo et al. on Feb. 8, 1983. A similar use of a moderate purity material is proposed in U.S. Pat. No. 4,200,492 to Armijo on Apr. 29, 1980. The ultrapure zirconium material described has been purified by iodide cells to produce so-called "crystal bar" material. This rather expensive crystal bar processing is performed after reduction and is discussed, for example, in U.S. Pat. No. 4,368,072 issued to Siddall on Jan. 11, 1983.
Hafnium has also been purified by the crystal bar process, and is used, for example, for control rod material in nuclear reactors.